1. Field of the Invention
This invention relates to earth boring drill bits, and in particular to a fixed cutter drill bit having cuffing elements positioned so as to facilitate directional drilling.
2. Description of the Related Art
Until quite recently, drill bit designers worked primarily on designing drill bits which would drill straight holes through the earth. More recently, designers have been working on bit designs which, when used in conjunction with suitable downhole equipment, can be steered to permit directional drilling. In directional drilling, it is important to ensure that the drill bit does not wander from the desired path. In addition, the bits must be easy to steer and be able to hold a horizontal drilling trajectory.
There are two common ways to steer a drill bit. One method is to use a downhole motor to rotate the drill bit, the motor and drill bit being mounted upon a drill string including an angled bend. In such an arrangement, the direction of drilling is dependent upon the angular position of the drill string. In use, the drill string is rotated until the drill bit is pointing in the desired direction. The drill string is then held against further angular movement whilst drilling in the desired location takes place. This steering technique is sometimes known as xe2x80x9cpointing the bitxe2x80x9d.
An alternative steering technique is known as xe2x80x9cpush the bitxe2x80x9d. In this technique, the drill bit is rotated continuously. Associated with the drill bit is a unit designed to permit the application of a side load to the drill bit to cause the direction of drilling to deviate from the straight path it would otherwise follow. U.S. Pat. Nos. 5,265,682; 5,553,679; 5,582,259; 5,603,385; 5,685,379; 5,706,905; 5,778,992; 5,803,185 all incorporated herein by reference for all they disclose describe a unit suitable for use in a push the bit type steerable drilling system.
A known drill bit suitable for use in a steerable drilling system of the push the bit type has a leading face from which a plurality of blades upstand, each blade carrying a plurality of cutting elements. Each blade terminates in a gauge pad. In a typical drill bit, the gauge pads are not provided with cutting elements, but may be provided with inserts designed to improve the wear resistance of the gauge pads. It has been found, however, to be advantageous in a push the bit type system to provide the gauge pads with cutting elements. One disadvantage, however, of applying cutting elements to the gauge pads is that there is a tendency for a wellbore formed using the drill bit to drop. This is due to the gravitational effects experienced by the drill bit, the effect being greatest where a horizontal bore is to be drilled, the gravitational effects applying a side loading to the drill bit causing the cutters of the gauge pads located at the lower side of the bit at any given time to become active in drilling the borehole.
It is an object of the invention to provide a drill bit particularly suitable for use in a steerable drilling system of the push the bit type.
According to the present invention there is provided a drill bit for drilling a borehole, the drill bit comprising a bit body having an axis of rotation, a leading face, a plurality of blades upstanding from the leading face, at least one of the blades terminating in a gauge pad having a gauge surface arranged, in use, to face a wall of the borehole, the gauge surface being devoid of cutting elements, the gauge surface terminating at an end thereof remote from the blade at a junction with a gauge pad end wall, wherein the gauge pad carries a single cutter having a face and a cutting edge located radially inward of the gauge surface, and wherein the junction of the gauge surface and the gauge pad end wall crosses, radially, between the face of the cutter and the wall of the borehole.
Preferably each blade terminates in a similar gauge pad, each gauge pad carrying a single cutter. Each cutter conveniently comprises a table of a superhard material bonded to a substrate. The superhard material preferably comprises diamond.
The cutting edge is preferably spaced radially inward of the gauge surface by a distance greater than about 0.15 mm, and preferably between about 0.2 mm and 0.5 mm.
A line drawn between the cutting edge and the junction conveniently makes an angle with the axis of the bit of less than about 0.1xc2x0.
According to another aspect of the invention there is provided a drill bit for drilling a borehole, the drill bit comprising a bit body having an axis of rotation, a leading face, a plurality of blades upstanding from the leading face, at least one of the blades terminating in a gauge pad having a gauge surface arranged, in use, to face a wall of the borehole, the gauge surface being devoid of cutting elements, the gauge surface terminating at an end thereof remote from the blade at a junction with a gauge pad end wall, wherein the gauge pad carries a single cutter having a face and a cutting edge located radially inward of the gauge surface, the radial spacing of the cutting edge from the gauge surface being greater than about 0.15 mm.
According to yet another aspect of the invention there is provided a drill bit for drilling a borehole, the drill bit comprising a bit body having an axis of rotation, a leading face, a plurality of blades upstanding from the leading face, at least one of the blades terminating in a gauge pad having a gauge surface arranged, in use, to face a wall of the borehole, the gauge surface being devoid of cutting elements, the gauge surface terminating at an end thereof remote from the blade at a junction with a gauge pad end wall, wherein the gauge pad carries a single cutter having a face and a cutting edge located radially inward of the gauge surface, and wherein a notional line between the junction and the cutting edge makes an angle with the axis of rotation of less than about 0.1xc2x0.
According to another aspect of the invention there is provided a drill bit for drilling a borehole, the drill bit comprising a bit body having an axis of rotation, a leading face, a plurality of blades upstanding from the leading face, at least one of the blades terminating in a gauge pad having a gauge surface arranged, in use, to face a wall of the borehole, the gauge surface being devoid of cutting elements, the gauge surface terminating at an end thereof remote from the blade at a junction with a gauge pad end wall, wherein the gauge pad carries a single cutter having a face and a cutting edge located radially inward of the gauge surface, and wherein an axial position of the junction of the gauge surface and the gauge pad end wall lies between an axial position of an edge of the cutter face closest to the blade and an axial position of an edge thereof furthest from the blade.
According to a further aspect of the invention there is provided a steerable drilling system for use in the drilling of a borehole comprising a bottom hole assembly (BHA) adapted to permit control over the direction of drilling of a drill bit, the drill bit comprising a bit body having a plurality of blades, at least one of the blades terminating in a gauge pad having a gauge surface devoid of cutting elements, the gauge pad having, at an end thereof remote from the blade, a gauge pad end wall, the gauge pad carrying a single cutter located inward of the gauge surface and such that an axial position of a junction between the gauge surface and the gauge pad end wall lies between an axial position of an edge of the cutter closest to the blade and an axial position of an edge of the cutter furthest from the blade.
The bottom hole assembly (BHA) may be of the xe2x80x9cpush the bitxe2x80x9d type including a bias unit arranged to apply a side loading to the bit. Alternatively, the bottom hole assembly may be of the xe2x80x9cpoint the bitxe2x80x9d type, including a downhole motor for rotating the drilling bit, and an angled member arranged to permit the bit to be supported in a desired orientation relative to an axis of the borehole.
It is envisaged that the drill bit may be used in a drilling system which is, in effect, a combination of the xe2x80x9cpush the bitxe2x80x9d and xe2x80x9cpoint the bitxe2x80x9d types, the system including, for example, a bias unit arranged to apply a side loading to a bent unit which carries a motor, the motor carrying the drill bit. In such an arrangement, when the system is to be used to drill a curve, the drill string is held against rotation with the bent unit holding the drill bit in the desired orientation whilst the motor drives the drill bit, and the bias unit is operated to apply a side loading to the bent unit and the drill bit.